Unmanned underwater vehicles (UUVs) can be used in a number of applications, such as undersea surveying, recovery, or surveillance operations. However, supplying adequate power to UUVs for prolonged operation can be problematic. For example, one prior approach simply tethers a UUV to a central power plant and supplies power to the UUV through the tether. However, this clearly limits the UUV's range and deployment, and it can prevent the UUV from being used in situations requiring independent or autonomous operation. Another prior approach uses expanding wax based on absorbed heat to generate power, but this approach provides power in very small amounts, typically limited to less than about 200 Watts (W) at a 2.2 Watt-hour (WHr) capacity. Yet another prior approach involves using fuel cells in a UUV to generate power, but fuel cells typically require large packages and substantial space. It is also often necessary or desirable to provide ballast systems on UUVs in order to help stabilize the UUVs and provide buoyance management during use.